The invention relates to a control device for a motor of a type having at least two motor parts which interact with each other via a force transmission interface and are movable relative to each other, a sensor for determining the state of movement of the motor parts, and a controller for controlling the state of movement of the motor in response to signals of the sensor. The present invention further relates a process for controlling a motor.
Control devices for operating motors are generally known. For this purpose, for example, a sensor determines the state of movement and/or the position of the motor which is then supplied to a control device that operates the motor depending on the difference between the desired value and the actual value of the motor parameters. In this manner, precise adjustment of time-dependent set parameters, such as position, speed or acceleration can be realized.
Continued technical advancements, however, necessitate a more precise control of, e.g., electromotors, but also of hydraulic motors and combustion engines.
Control devices of modern automatic production machines, in particular automatic bonding machines, pick-and-place robots, milling machines and the like, have to satisfy different and sometimes contradictory requirements.
For example, a pick-and-place robot that places electronic components on printed circuit boards, has to realize a point-to-point movement in which, on one hand, the component must be placed at pinpoint accuracy and, on the other hand, the point-to-point movement has to be executed at a high speed, so that at a consequence of the acceleration and deceleration oscillations are caused in the system which can adversely affect the placement accuracy. This applies also to modern milling machines where a relatively small milling head is used for differently sized milling openings and contours. Precise placement requires here also a high speed and acceleration, respectively, of the milling head.
Translational drives for conventional machine tools, e.g. milling machines, and of pick-and-place robots use ball roller spindle drives in which an electromotor drives a ball roller spindle which then moves a tool platform or a workpiece platform. In this design, the motor is controlled by a rotation sensor located on the motor and also by a linear scale located on the tool platform.
All mechanical components of the ball roller spindle drive, however, have been found to adversely affect the controlled system, i.e. time delays due to the elastic properties of the spindle, the oscillations of the workpiece or of the tool platform, etc., cause phase rotations in the controlled system which can lead to an oscillation problem.
This problem can be partially alleviated by using direct drives, i.e. electromagnetic linear motors, since a linear direct drive eliminates, e.g., the mechanical component of the ball roller spindle. It should be noted, however, that all mechanical elements, e.g. the suspension of the milling head, of the workpiece platform etc., adversely affect the controlled system, although to a lesser extent. Moreover, the operation of the control circuit itself also becomes an important factor. In connection with a speed control, an actual speed signal is typically derived from an actual position signal and compared with a desired speed signal. The differentiation with respect to time introduces a system delay which manifests itself as a phase delay or phase rotation and adversely affects the controlled system. Furthermore, the position data are typically measured as discrete data points, resulting in quantization noise which is amplified during differentiation with respect to time. This situation is even more pronounced when the acceleration is controlled.
The so-called cascade control where several controllers, for example a position control, a speed control and an acceleration control, are connected in series, experiences similar problems.
The accuracy of the control can be improved by using a so-called pilot control wherein, for example, an additional speed pilot control signal which is derived from the desired values, is supplied to the speed controller. However, the above-stated problems associated with phase rotation, phase shift and delay remain.